A photolithography tool for printing semiconductor integrated circuits is a typical representative of high precision production equipment. On the one hand, positioning accuracies for the stages carrying the mask and the object (wafer) carrying multiple images of the printed circuits are measured in nanometers (nm), but on the other hand, the productivity requirements call for very high velocities and accelerations of movements of the massive stages and for short settling times after the movements terminate (stop).
The photolithography tools have extremely complex structures comprising the stage units, their driving motors, intricate and high precision illumination and focusing systems, and numerous precision sensing and measuring devices (usually, interferometers). A typical stage unit comprises the stage proper supporting the object (substrate) or the mask (reticle), stage frame or base plate supporting the stage motion, and other components supporting ancillary and corrective movements and/or measuring devices. As a result, the structures have many joints and cannot be made very stiff, thus resulting in high sensitivity to vibrations, e.g. see E. I. Rivin, Passive Vibration Isolation, ASME Press, 2003. To reduce damaging effects of the inevitable external vibrations on the precision semiconductor devices, extremely stringent and expensive to satisfy requirements to vibration levels of the floors and other supporting structures for the photolithography tools are mandated. The allowable vibration amplitudes of the floor are in fractions of one micrometer (˜0.01–0.12 μm or 10–120 nm), e.g. see the above cited book.
The above two paragraphs reveal a design contradiction which is critical both for designing and for using the high precision photolithography tools and the like: Advancement of the integrated circuit technology requires reducing the line widths (already counted in nanometers), thus increasing sensitivity of the tools to external vibrations, while productivity enhancement requirements call for intensification of the positioning process of the stages, thus for higher programmed accelerations and, consequently, greater dynamic loads generated within the tool itself and creating dynamic (inertia) forces causing very intense vibrations, far exceeding vibrations transmitted from the floor. The dynamic forces are further intensified by the trend for increasing the wafer size (e.g., from 200 mm to 300 mm) which leads to increasing sizes and masses of the object stages. The dynamic force amplitudes exceeding 1 KN are quoted in Kwan, Y. B. P., Loopstra, E. L., “Nullifying Acceleration Forces in Nano-Positioning Stages for Sub-0.1 μm Lithography Tool for 300 mm Wafers,” in Proceed of the 15th Annual Meeting of ASPE, 2000, pp. 525–528.
One way of protecting the sensitive photolithography tools and similar structures from such intense dynamic loads can be achieved by using so-called force or reaction frames, e.g. as described in U.S. Pat. Nos. 5,260,580 and 6,271,640. These patents teach attachment of stationary parts of the stage-driving linear motors to a structure rigidly mounted on the floor or on other supporting structure. If there are several motors driving a particular stage, the stator of the primary motor performing the relatively long stroke/high acceleration programmed movement of the stage (rather than of the motors providing much slower small stroke correction movements associated with low accelerations and dynamic inertia forces) is attached to the reaction frame. The first (moving) part of the primary motor is connected to the driven stage which is, in its turn, connected to the tool structure (so-called metrology frame) via pressurized fluid (usually, air) bearings or guides having extremely low friction. The tool structure is dynamically isolated directly from the reaction frame and/or from the floor to which the reaction frame is mounted by low stiffness vibration isolators. As a result, the high dynamic forces do not noticeably disturb the precision metrology frame. While the metrology frame is isolated from the intense dynamic forces, the complexity of the apparatus is increased by addition of massive and bulky force frame which has to be precisely aligned with the metrology frame. Transmission of the intense reaction force to the floor excites floor vibrations which are undesirable not only for the tool to which the reaction frame belongs, even with vibration isolators, but also to other equipment units mounted on the floor. To alleviate this effect, extremely high stiffness of the floor is specified, in the range of 1−2.5×105 N/mm. Needless to say, such floors are extremely expensive. Another shortcoming of this approach is the fact that the relatively long stroke movements of the massive stage unit cause shifts of the center of mass of the metrology frame and subsequent undesirable tilting of the metrology frame. Compensation of the tilting in the system with the reaction frame requires a sophisticated and expensive servo-controlled system, e.g. as described in U.S. Pat. No. 5,844,666.
Another approach to reducing dynamic forces acting on the metrology frame and caused by acceleration/deceleration of the stage is described in U.S. Pat. No. 6,449,030 to Kwan and shown in FIG. 1 (the Prior Art) with self-explanatory labels from the above cited article by Kwan, et al. Long-stroke (pre-programmed) “LoS” motion of the reticle stage having mass mf(labeled “LoS Frame” in FIG. 1) in the direction of arrow is accompanied by the motions in the opposite direction (shown by arrows) of Balance Masses 1 (mass mb1) and 2 (mass mb2) separated from the stage by low-friction LoS X and LoS Z Bearings. Two LoS Y Actuators (linear motors) have coils on the stage and magnets on the Balance Masses, thus applying equal but opposite forces to the stage and to the Balance Masses. Acceleration/velocity time histories of the Balance Masses may be different from those of the stage and from each other depending on ratios mf/mb1, mf/mb2, mb2/mb1. This approach does not require the reaction frame since the dynamic forces cancel out within the system. Also, the center of mass remains essentially stationary with possibly minor deviations due to variations of friction forces in the multiple bearings. One disadvantage of this approach is its design complexity and, consequently, high costs. There is a need for two precisely machined balance masses and for supporting surfaces for them on the machine frame, for numerous bearings, and for a complicated alignment procedure between the stage, the balance masses, and the machine frame. The correction short-stroke (SS) motions of the reticle table, effected by SS X-, SS Y-, and SS Z-actuators, are not balanced.
U.S. Pat. No. 5,815,246 discloses a system having a first balance mass performing translational motions and a second balance mass performing rotational motion and driven by a rotation motor. This concept is also characterized by excessive design complexity.
International Patent WO 98/28665 to Loopstra, et al, discloses a balancing system for two stages simultaneously, one stage in the operating position and another stage residing on the measuring station This system is characterized by one large balance mass providing balancing action for both X and Y motions of both stages (X and Y being coordinate directions in the horizontal plane and perpendicular to the vertical Z axis). However, since the two stages have different motion histories, the residual dynamic forces may still have harmful effects. Also, the center of mass may shift in this system and the resulting tilting of the tool is not fully compensated.
These and other disadvantages are addressed and overcome by the present invention.
Although specific references are made in this Specification to the photolithography apparatus (tool) for manufacturing of integrated circuits, it should be understood that the like apparatuses may have many other applications. They may be employed in manufacturing of integrated optical systems, magnetic domain memory systems, liquid crystal display panels, thin-film magnetic heads, etc. Accordingly, the use in the text of such specific terms as “reticle”, “wafer”, etc. should be considered as representing more general terms such as “mask”, “substrate”, etc. The terms “radiation” and “beam” are representing all types of electromagnetic radiation or particle streams such as, but not limited to, ultraviolet, X-rays, electrons, and ions.